Waste heat recovery systems can make available for use energy in exhaust gases that would otherwise be lost. When incorporated in a vehicle with an internal combustion engine, waste heat recovery systems add certain advantages. For example, the waste heat recovery system can be designed to recover heat from the EGR (exhaust gas recirculation) system, which reduces the cooling load on the engine cooling system.
In addition, a waste heat recovery system can extract useful energy from the exhaust gas exiting the tail pipe or exhaust stack, which would otherwise be lost to the environment.
The invention provides a method and apparatus for improving the recovery of waste heat from an internal combustion engine exhaust. The recovery of additional energy improves efficiency of the system as a whole.
In addition, according to one aspect, the invention improves the operation of the waste heat recovery system itself, by, for example, reducing the cooling load on a waste heat recovery system condenser.
Further, by preheating the working fluid before it enters an exhaust gas heat exchanger, the exhaust gas remains at a higher temperature, avoiding condensation in an exhaust gas flow.
A waste heat recovery apparatus for an internal combustion engine may include a working fluid circuit on which are connected an expander for converting heat energy to mechanical or electrical energy, a condenser, a pump for moving the working fluid through the circuit, and a first heat exchanger for transferring heat from the internal combustion engine exhaust to the working fluid.
According to the invention, the working fluid circuit includes a first heating line and a second heating line in parallel with the first heating line. A first heat exchanger or boiler is connected in the first heating line and operatively connected to an exhaust gas conduit carrying exhaust gas to a stack outlet or tail pipe.
A second heat exchanger is provided in the second heating line by connecting the working fluid circuit to an exhaust gas recirculation cooler configured to transfer heat to the working fluid from the exhaust gas being recirculated to the engine air intake.
A valve downstream of the pump and responsive to an enthalpy demand of the waste heat recovery system controls the flow and distribution of working fluid into the first heating line and second heating line.
Alternatively, a pump may be positioned, on each of the first heating line and the second heating line, both pumps downstream of a junction where the working fluid circuit branches into the two heating lines, each pump being controlled responsive to the enthalpy demand of the system to pump a flow of working fluid into the respective heating line. The pumps may be variable speed or variable output pumps. Alternatively, each of the first heating line and second heating line may include a bypass arrangement, including a bypass line and valve, to carry a portion or all of the pump output and return it to the condenser, the pump inlet, or a reservoir tank.
Enthalpy demand takes into account the output demand of the waste heat recovery system, the available heat energy from the engine exhaust, and internal limitations of the system, such as a temperature limit of the working fluid, heat rejection load on the condenser, and other factors as will be appreciated by those skilled in the art.
According to another aspect of the invention, a recuperative heat exchanger is operatively connected to transfer heat from working fluid at the junction of the expander outlet and condenser-inlet (a higher-temperature, lower-pressure location) to the first heating line upstream of tire first heat exchanger (a lower-temperature, higher-pressure location).
Advantageously, the recuperative heat exchanger recycles some of the heat that would otherwise be rejected through the condenser as waste heat; thus, an effect of the recuperative heat exchanger is to increase the overall energy-conversion efficiency. In addition, the recuperative heat exchanger removes heat energy from the working fluid that would otherwise have to be removed by the condenser, thus lessening the cooling demand on the condenser.
In the description of the invention, the apparatus and method are described in connection with a Rankine cycle waste heat recovery apparatus, but it should be understood that the invention applies to other waste heat recovery or recuperation devices.